Such foldable wheelchairs are common knowledge from the prior art, for example from DE 20 2008 009 608 U1. This prior art makes use of a scissor-action element, also referred to as a cross-strut. The pivot axis of the scissor-action arms of the scissor-action element here is arranged parallel to the surface over which the wheels of the wheelchair stand on the ground. Therefore, in the case of the ground being arranged horizontally, the pivot axis common to the two scissor-action arms of the scissor-action element is arranged horizontally. When the wheelchair is folded together, this pivot axis moves upwards, that is to say in a vertical plane. The lower ends of the scissor-action arms are mounted in a rotatable manner on lower horizontal tubes of the side frames. A respective seat tube is welded horizontally onto the upper ends of the scissor-action arms. These horizontally welded-on seat tubes rest, in the opened-out state of the wheelchair, on bearings which are adapted to the seat tubes and are fastened on upper, horizontal tube portions of the seat frames. During the folding operation, the horizontally welded-on seat tubes move freely upwards, while the side frames move towards one another. A respective lug, which is connected in a rotatable manner both to one scissor-action element half and to the corresponding side frame, ensures that the side frames, during the folding operation, are guided parallel to one another and the seat tubes of the scissor-action element are positioned precisely in the bearings.
Also known in practice are the “AVANTGARDE” series of foldable wheelchairs from Otto Bock HealthCare GmbH, D-37115 Duderstadt, the “EASY” series of foldable wheelchairs from Sunrise Medical GmbH & Co. KG, D-69254 Malsch and the “Traveller4you” series of foldable wheelchairs from PROACTIV Reha-Technik GmbH, D-72359 Dotternhausen.